Line symbols in ArcMap appearing and disappearing when zooming in and out

[AmyNewsam]

I'm having some strange glitch where a line symbol partially disappears at different zoom levels.  I thought perhaps it was just a display issue, but when I exported the layout, it was still missing part of the symbol.

This is what it is supposed to look like (line highlighted)

and here are two zoom levels:  (it's not the actual zoom level that determines which parts of the line show up, it seems random.)

  

I imported to Pro and it worked just fine, but I'm just curious if this is something others have experienced and if there's an explanation.  I usually work in Pro, but have a couple of clients who require ArcMap.